IxD in the olden Days

I'm curious about the way this discussion is started with the word "OK"
Could anyone tell me the Subtleties or reasons behind this. I'm more
interested with the cultural aspects of this action. In my culture, and
in the olden days of paper letters, people used to mark the envelope and the
page corners with yellow color of turmeric. It used to symbolize the good
news inside. Similarly, the death used to be marked by black color.

I thought this was a form of interaction design in olden days. Are there any
such designs in other cultures. [not restricted to letters] Please comment.